Alkali metal aluminosilicates, both crystalline (zeolites) and amorphous, are effective detergency builders which can be used to replace sodium tripolyphosphate (STP) in detergent powders, but they do not possess an ability comparable to that of STP to contribute to the structure of a spray-dried powder. Alkali metal silicates are frequently included in detergent powders as structurants, to reduce washing machine corrosion and to increase alkalinity. It is well known, however, that if aluminosilicate and silicate are together in a detergent slurry they can interact unfavourably: agglomeration of the aluminosilicate occurs to give powders containing large particles which are slow to disperse in the wash liquor, giving reduced washing performance.
U.S. Pat. No. 4,129,511 (Lion) discloses a method of making a detergent composition containing 5 to 20% sodium silicate and 0.5 to 5% aluminosilicate by subjecting a detergent slurry containing silicate and an aluminosilicate slurry to spray-drying separately within the same drying space.
GB No. 2 013 707B (Unilever) discloses a process in which an aqueous solution or dispersion of sodium silicate is injected into a silicate-free or low-silicate, aluminosilicate-containing detergent slurry after it leaves the crutcher but before atomisation at the spray-nozzle. This measure reduces the time of contact between silicate and aluminosilicate in the slurry, but does not eliminate such contact.
EP No. 10 247B (Henkel KGaA) discloses a process in which silicate is omitted from the slurry, and instead is subsequently admixed as a solid with the spray-dried powder. The slurry contains aluminosilicate, surfactant and certain organic sequestrant builder materials, while the silicate is postdosed in the form of a powder having a Na.sub.2 O:SiO.sub.2 mole ratio of 2.0 to 2.2, a water content of 15-23% by weight and a high water solubility. Other ingredients unsuitable for spray-drying, for example, certain nonionic surfactants, may also be postdosed.
Powders made by this process exhibit improved washing performance, since the aluminosilicate is carried through into the powder, and into the wash, in the form of small particles. The physical properties of these powders tend, however, to be poor and the powder strength low.
Our European Patent Application No. 87 302 911.0 filed Apr. 3, 1987 discloses an improved process wherein solid sodium silicate is postdosed to a spray-dried base powder containing an additional powder structurant which is a polymeric material. Our European Patent Application No. 87 303 159.5 filed on Apr. 10, 1986 discloses and claims a similar process wherein the spray-dried base powder contains a succinic acid salt as an additional structurant.
In these processes involving postdosing of solid sodium silicate to a spray-dried base powder, it is essential that the sodium silicate be of high quality and have rapid dissolution characteristics: such high-grade sodium silicate is expensive. Handling of this rather dusty material in the factory can also cause safety problems.
We have now discovered an alternative process for introducing sodium silicate into aluminosilicate-built powders while avoiding contact, and unfavourable interaction, between the two components in the slurry. The process results in powders containing characteristic composite granules.